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S11 Cable Time Domain

Question asked by BDumke on May 17, 2007
Latest reply on May 17, 2007 by odanzy
Did an S11 Cal on an Agilent 8753E, and measured a length of Andrew LDF4-50A 1/2" Heliax terminated in a good Weinschel 50 Ohm termination.   So both the source (network analyzer Port 1) and load matching were pretty good. 

When I look at the time domain response there is a raised (noise?) floor between the source and load reflections.  What is this floor caused by? 

Is it due to the characteristic impedance of the cable being off from 50 Ohms? 

It does not change significantly with the bandwidth or number of points settings.  It is the same for 3700 Hz BW with 201 points as well as 10 Hz BW with 1601 points. 

I see similar time domain displays with other lengths of heliax cable although the shape of the floor is somewhat different.

See attached jpg files for the frequency and corresponding time domain response.